This invention relates to controlling a coating head that is configured to exert a nip pressure to a fibrous web to be coated, while the fibrous web is supported by a backing roll.
Monitoring and controlling the behavior of the coating blade, as well as its position in relation to the fibrous web to be coated, is important, when the operation of a coating head and the machine of which it forms a part is to be optimized. Similar problems and objectives sometimes come up in controlling the rod load in rod coating or film transfer coating.
The Japanese patent publication JP 5015833 (A) (also expressed as JP 05015833 A), published on Jan. 26, 1993, and assigned to Mitsubishi Paper Mills Ltd., discloses an arrangement for monitoring and adjusting the coating profile. It suggests using a row of controllable actuators to move the profiler bar that exerts transverse pressure to the coating blade, and integrating an electronically readable pressure sensor to each actuator. According to said publication, the electronically collected readings of the pressure sensors can be used to represent the pressure profile, and if the actuators are machine-operated, even to implement some kind of automatic feedback from the pressure sensor readings to the actuators.
Another prior art solution is known from the Japanese patent publication JP 2005219054, published Aug. 18, 2005, and assigned to Mitsubishi Paper Mills Ltd. It suggests monitoring the coating profile with a BM meter (Basis Mass; more commonly referred to as Basis Weight or BW) located downstream from the coating head, calculating a floating average of the measurements, and using them to automatically adjust the actuators that move the profiler bar.
Yet another known solution is the use of position sensors in the actuators to describe the physical location of each actuator that supports the profiler bar.
The known solutions involve certain drawbacks. A measurement device integrated in the actuator, like in JP 5015833 (A), gives readings that describe primarily the stress distribution in the profiler bar, from which it is not possible to unambiguously derive all desired characteristics of the coating blade. A scanning profile measurement, like in JP 2005-219054, is not capable of appropriately telling cross-directional phenomena from machine-directional ones, and suffers from the inherent delays related to the physical distance between the coating head and the measurement frame as well as the time it takes to scan the whole width of the fibrous web. Position sensors give information only about the location of the actuators, which does not take into account e.g. the possibly uneven abrasion of the blade or the cyclic machine-directional interference resulting from a slight eccentricity of the backing roll.